1. Field of the Invention
The present invention relates to an apparatus and method for detecting and isolating faults on a telephone line system, and more particularly, the invention relates to a novel line monitoring circuit and method for isolating a utility-provided central office line from an on-premise line upon detecting a line-to-ground fault or an extraneous AC voltage fault.
2. The Prior Art
Until recently, all of the equipment used on telephone systems was provided exclusively by closely regulated telephone utilities. However, recent changes in the law now make it possible for customers to provide their own telephones, PBX systems, wiring and the like. Accordingly, privately provided and privately installed subscriber-terminal equipment is fast becoming a major segment of telephone systems.
However, the variety of different devices which are now available for installation at the subscriber terminal of a telephone system has raised some problems. For example, telephone utility companies have raised questions with respect to how their equipment will be protected from faults occurring on the subscriber-terminal portion of the telephone system. These faults may typically occur in the form of either line-to-ground faults or extraneous AC voltages impressed upon the telephone line. Such faults occurring in the equipment or wiring installed on the subscriber-terminal portion of a telephone system may adversely affect the operation of the utility-owned central-office line and may thereby adversely affect other users who are connected to the telephone network.
Furthermore, since the public utility companies are no longer responsible for servicing and maintaining these privately-provided facilities, the question has arisen as to how the customer will know whether a fault or defect has occurred on the utility-provided portion of the line, in which case a serviceman from the utility company should be called, or whether the fault or defect has occurred on the privately-provided facilities, in which case a repairman from the private sector of the industry should be called. Thus, in order to avoid redundant and conflicting calls to both utilities and private servicemen upon the occurrence of a fault, some way is needed to help differentiate, at the subscriber-terminal, between faults which occur on the utility-provided central-office line and those which occur on the privately-provided facilities.
Although there have been some prior attempts to solve these problems, these prior attempted solutions have relied upon telephone line monitoring equipment which must operate and function based upon the sensing of electrical potentials with respect to ground potential at the central office terminal of the telephone system rather than at the subscriber terminal. Accordingly, these prior attempted solutions have not solved the problem of how to differentiate for a customer, when a fault has occurred at the subscriber terminal, what type of fault has occurred and on what portion of the telephone system the fault has occurred (i.e. whether it has occurred on the utility-provided central-office line or whether it has occurred on the privately-provided facilities).
Furthermore, prior attempted solutions to these problems have not provided a sufficiently wide range of capability in terms of protecting against a variety of fault conditions. Typically, a telephone line is comprised of two conductors, one of which is designated a ring conductor, and one of which is designated a tip conductor. Additionally, at the subscriber premise there is provided by the utility company a ground terminal and a grounding conductor. The grounding conductor is typically connected to the water pipe system or to the neutral conductor or ground conductor of the local electrical distribution system. This grounding system provides a path for fault currents caused by lightning or resulting from high-voltage power lines that may fall onto the telephone lines, as during a storm.
Further fault conditions may also occur because of a line-to-ground fault on either of the ring or tip conductors, or through the imposition of extraneous AC voltages on either the tip or ring conductors. These latter mentioned fault conditions are typically sufficiently low that lightning protectors will be ineffective. However, they are nevertheless high enough to pose a hazard to personnel and to damage the facilities of the telephone utility company.
Presently, there is no protective device available in the industry which is capable of protecting against both line-to-ground faults on either of the ring or tip conductors and which can also protect against extraneous, hazardous AC voltages which may be impressed upon the ring or tip conductors through contact with power lines. Furthermore, although there are available high-voltage protective devices that provide protection against extraneous voltages on the order of 300 volts AC and above, the most prevalent potential exposure at the on-premise location is to voltages on the order of 120 volts AC. This 120 volts AC is an extremely hazardous level of voltage, yet there presently is no protective device in the industry that copes with this potential problem.
Accordingly, what is needed is a line monitoring device which may be installed at the subscriber terminal and which can isolate the central-office line from the privately-provided facilities at the subscriber terminal in the event of a line-to-ground fault on either of the ring or tip conductors or in the event of an extraneous AC voltage fault on either conductor. Furthermore, a line monitoring device is needed which is capable of indicating to the customer at the subscriber terminal (1) when a fault has occurred, (2) what type of fault has occurred, and (3) on what portion of the system the fault has occurred (i.e. whether the fault has occurred on the utility-provided central-office line or whether the fault has occurred on the privately-provided facilities at the subscriber terminal).